In multicolor web-fed printing press systems, a web of material (e.g., paper) is sequentially driven through a series of printing units, each comprising a plate cylinder and a print cylinder (blanket cylinder). Each blanket cylinder contacts the web in sequence and applies a different color of ink thereto, which colors cooperate to imprint a multicolor image on the web. As the web exits the printing units, the ink is still wet, and thus subject to smearing. Accordingly, before further processing, the web is typically routed through a drying unit to dry the image, heating the web to evaporate various solvents in the ink, then to a chill roller unit to cool the web and set the ink.
To provide an accurate and clear multicolor image, the rotational and lateral position of each blanket cylinder must be precisely aligned, i.e., proper registration of the respective colors must be maintained. Historically, the registration of the various print cylinders in multicolor systems was maintained manually. A pressman would examine signatures (printed images) at the output of the press, and manually enter estimated lateral and rotational offset values into an electromechanical register control system to effect the necessary corrections. Maintenance of color registration in such systems requires the constant attention of the pressman since registration is often lost due to a number of uncontrollable variables in the web material and press hardware.
Automatic registration control systems for multicolor web-fed printing press systems are, in general, known. For example, commercially available closed loop register control systems utilize an optical scanning device cooperating with register marks printed on the web by the individual cylinders, to provide position feedback information indicative of the registration of the respective print cylinders relative to a designated reference print cylinder. More particularly, each print cylinder produces a specific register mark forming part of a register pattern. The optical sensor generates a signal indicative of the register pattern, which is analyzed to determine the lateral and rotational registration of the respective print cylinders vis-a-vis the reference cylinder. Registration error signals, produced in accordance with the registration pattern, are employed to effect position correction of the respective print cylinders. Examples of such systems are described in EPO Application No. 87 104 973.0, filed April 3, 1987, and U.S. Ser. No. 849,095, filed July 2, 1986 by the present inventor, both applications commonly assigned herewith.
Optimal scanning accuracy may be achieved when the web is scanned under conditions yielding relatively little web "weave" (spurious lateral movement of the web, e.g., movement transverse to the direction of web travel, in the plane of the web) and "flutter" (spurious movement of the web in a direction perpendicular to the plane of the web). Preprinted control marks are preferably as small and unobtrusive as possible. However, the ability of the scanner to accurately detect the presence and position of a mark tends to be inversely proportional to mark size; the smaller the mark, the more likely that misregistration or web weave will take the mark outside of the field of view of the scanner. While use of a small and unobtrusive mark can be facilitated by use of a line scanner, as in the aforementioned Sainio U.S. Ser. No. 849,095 (RGS IV), substantial web weave may cause the scanner to lose track of the mark, necessitating reacquisition of the mark by the registration system or, in some cases, physical translation of the scanner to bring the mark back into the field of view of the optical scanner. Reacquisition of the mark can require a significant amount of time in the context of system operation, thereby impairing scanning efficiency.
In addition, optical scanners tend to have a relatively limited depth-of-field, i.e., they are capable of accurately sensing only those images within a predetermined range of distance from the scanner, typically on the order of approximately 0.025 inches. Thus, web flutter in the vicinity of the scanner should be maintained within the limits of the scanner depth-of-field. In prior art systems, flutter is typically maintained within acceptable limits by physically restraining the web, e.g., scanning the web as it wraps around an idler roller, or the like, or in the vicinity of such a wrap.
It is desirable that misregistration be detected as quickly as possible after printing, i.e., that the web be scanned as early in the process after the printing operation as possible. At high web speeds (e.g. 2000 feet per minute), relatively short delays in detecting misregistration can cause considerable wastage.
A principal source of web flutter is observed at the line of contact between the web and the final print cylinder as the web leaves the printing stage. The ink applied to the web by the print cylinder is tacky when moist, causing the web to adhere to the outer circumference of the print cylinder. In regions of high image density, the adhesion is relatively strong; in regions of low image density, the adhesion is relatively weak. Localized fluctuations in web tension as the web is pulled from the print cylinder surface cause the web to flutter with an amplitude in the range of about 3/16 to 1/4 inch in the vicinity of the final print cylinder, far beyond the maximum depth-of-field variations tolerated by commercially available scanners (e.g. 0.025 inches). Accordingly, to maintain flutter amplitude within the depth-of-field limits of the scanner, flutter amplitude must be reduced by approximately a factor of ten between the point at which the web leaves the print cylinders and the point at which the web surface is scanned.
Conventional web stabilizing techniques, which require physical contact with the web, are not suitable for use upstream of the chill roller; to avoid marring the printed image, physical contact with the web surface is not advisable until after the ink has fully dried. When the web emerges from the dryer, the flutter amplitude is typically less than 0.010 inches, well within the acceptable depth-of-field range of available scanners. The drying unit typically supports the horizontally oriented web using pressurized air simultaneously directed at the upper and lower surfaces of the web. This tends to dampen the flutter interjected by the printing units, effectively stabilizing the web during the drying operation. However, changes in the drying air pressure can cause the web to shift up or down relative to the scanner, resulting in unwanted low frequency depth-of-field variations. Moreover, various web characteristics can cause the web to dry at different rates along the length thereof, resulting in non-uniform shrinkage or expansion of the web. This can result in web weave, on the order of about 1/2 inch. This is compounded by periodic cleaning of the blanket cylinders (known as a "blanket wash"). A blanket wash obliterates registration marks, and often makes the web weave; the marks disappear, then reappear in a different lateral location due to the web weave caused by the blanket wash. Thus, the register control system almost invariably loses "track" of the mark, and must reacquire the mark after a blanket wash. This, of course, delays correction of misregistration. In addition, a web typically travels between 100 and 160 feet between the point at which the web emerges from the printing units and the point at which the web emerges from the dryer. Considerable wastage results from the delay in detecting misregistration. Thus, a technique is needed for stabilizing the web, without contact, prior to the drying operation.
Several mechanisms which turn or support the web without touching it, using a cushion of air, are commercially available. An example is the Tec Systems Tec-Turn(R), which turns a web of paper approximately 90 degrees upward into an overhead dryer. The Tec-Turn unit is adequate for turning, but the distance from the air outlet to the paper may vary from a few hundredths of an inch to 1/4 inch, depending on paper tension and air pressure. This is adequate for turning the paper but inadequate for keeping the paper within the practical focusing range of a scanner.
Attempts have been made to increase the depth of focus of scanners employing complex optics, thereby facilitating scanning under conditions of high amplitude flutter. For example, the Caligraph System by Bertin may be mounted after the printing groups and before the drier. Such systems, however, have tended to be impractical, overly bulky, and expensive.